The amount of air lost from leaving a system pressurized to atmosphere can be staggering.
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Fig. 1. Cutaway of packed spool valve shows flow being partially exhausted when valve shifts positions. |
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Fig. 2. Cutaway of poppet valve shows how flow can briefly be exhausted when valve shifts positions. |
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| Fig. 3. Cutaway of spool and sleeve type valve shows flow paths in various positions. |
The amount of air lost from leaving a system pressurized to atmosphere can be staggering. Logically, if a system is not being used, the air should be turned off as air will leak out any possible path of low resistance, whether it be past the dynamic seals of a rubber packed valve, a fitting thread, a cylinder rod seal, or from many other possible locations.
A shut-off or lock-out/dump valve is one of the few places in a pneumatic circuit where a "bubble tight" dynamic seal valve can make a system more efficient, but only if it is actually used to shut off the air supply when not in use. The benefit of using this type of valve as a shut-off or dump valve only holds true if the number of shifting cycles is limited.
Optimizing an application can mean different things to different engineers, so it is important to understand various concepts. For example, if speed is the primary goal, then the component with the least conductance (CV) must be identified and optimized to increase the overall CV of the system, thus improving speed.
It is also important to note that the term component includes all parts of the circuit — fittings, tubing, flow controls, regulators, etc. Any device or part of the circuit with an orifice is potentially a restriction, and less than optimal restrictions equal inefficiency. If component cost is the primary issue, then optimization would mean minimum component size to move the load at a given speed.
Most importantly, optimization equates to least air consumed. Determining ideal air pressures and components for optimal scfm consumption will result in a system requiring the least horsepower, minimizing energy consumption.
A question of leakage
All components are potential leaks, whether from a port, a fitting, a rod bushing on a cylinder, or across a dynamic seal in a valve or cylinder piston. Further, leakage implies waste, or a non-productive loss of air. It is also true that a spool-andsleeve construction valve will have as much as four times the air travel across a land as a bubble-tight style valve. However, the key difference is not the movement of the air, but understanding what that air movement means.
In a spool-and-sleeve valve, the thin film of air that travels across the lands (whether the valve is shifted or not) serves a purpose, and as such should not be referred to as leakage — it is consumption. The air travel in a lapped spool-and-sleeve valve supports the air-bearing effect that prevents any metal-to-metal contact. This frictionless fit is the secret to billions of cycles, compared with only a few million for dynamic seal valves. Many spool and sleeve valves installed in the 1950s are still running trouble-free today in their original applications.
Dynamic seal valves do suffer from leakage, as air traveling past the seals of a packed spool valve serves no benefit. If there is no benefit, then it should be considered waste or leakage, as it is in rubber packed spools or even ceramic style valves. Rubber packed poppet valves and rubber packed spools, can and do leak when stationary, especially after they have worn.
The term bubble tight is often used to describe the leakage rate of dynamic seal components. There is a common misconception that bubble tight equals zero leakage. This is not true — bubble tight only means there is so little leakage that a bubble of air does not break away when a product is cycled for a minute, submerged in a tank of water. A valve, cylinder, or any other device can leak as much as 50 cc/min and still be considered bubble tight. Further environmental conditions, such as temperature, moisture content, or dry vs. lubed service, all affect the wear and, thus, the leakage rate of a dynamic seal valve. However, while it may be true a quality lapped spool and sleeve valve may consume as little as 50 cc/min (it is more typical to be between 200-400 cc/min) compared to leakage of 10-50 cc/min in a bubble tight packed valve, the lapped spool and sleeve never changes, meaning it has a consistent consumption.
The critical assessment of a valve's efficiency is not when it is stationary, but when it shifts, because the function of a valve is to shift. A dynamic seal valve, whether a poppet or spoppet, will often suffer from a condition known as open crossover when a valve shifts.
Open crossover during rubber packed valve shifting can cost hundreds of times more waste than the thin film of air required to support an air bearing in a lapped spool-andsleeve valve. As seen in Figures 1 and 2, both the poppet and packed spool type construction suffer leakage every time the valve shifts, because the dynamic seal is not wide enough to seal across the gallery opening as the flow path changes from the A to the B position.
The loss of air during this period of open crossover is substantially greater than the consumption of a quality spool and sleeve. Further, the spool-and-sleeve style construction is designed to have a land that is wider than the gallery opening. Thus, there is no increase of air loss while the valve is shifting, making the spool and sleeve design one of the most efficient air valve designs with regard to minimal air leakage.
Think before selecting
Correct product selection does not end with the valve, however. Once optimum size and type of product has been determined, longevity should also be assessed. For example, a standard non-repairable cylinder may be able to move a load in an acceptable period of time and with a minimal amount of leakage past the dynamic seal on the piston and rod bushing. With any unguided or side load, however, wear begins with the first cycle. Side load will cause premature and excessive wear on both the rod bushing and piston seal, regardless of the materials of construction or style. This wear will cause leakage past the piston seal and rod bushing long before the cylinder actually fails or is determined to be in a non-working condition. Although this wear cannot be completely eliminated, the amount of wear and thus the leakage can be minimized through proper product selection.
Many times, a slide or gantry would be an appropriate solution, potentially adding millions of cycles to the life of the cylinder and maintaining the bushing and piston seal leakage to like new condition for years of operation.
Pneumatic optimization though intelligent circuit design, correct sizing, and informed component selection are the keys to energy conservation.